The present invention relates to medical radiography and, more particularly, to novel apparatus for producing a radiograph of acceptable quality while decreasing the required X-ray exposure therefore.
Known electrostatic X-ray image recording devices may utilize a photocathode as a direct X-ray photoelectron emitter, as described by K. H. Reiss and G. Lange, Phys. Med. Biol., 1973, Vol. 18, No. 5, pages 695-703. The device described therein is of the type consisting of a gas-filled chamber having a flat cathode spaced from a flat anode, with a voltage being applied between the cathode and anode. The flat cathode is made of, or coated with, a heavy metal. A sheet of insulative material, such as a plastic and the like, is positioned adjacent to the cathode-facing side of the anode. An X-ray image is directed onto the cathode for photoelectron emission responsive thereto; the photoelectrons are accelerated across the gas-filled gap and collide with gas molecules, forming electron-ion pairs and producing gain by an "avalanche effect" in the gas. As a result thereof, a pattern of electric charges is deposited on the plastic sheet in correspondence to the variable attenuated pattern of X-rays impingent upon the device. After exposure, the electrostatic charge pattern on the plastic sheet is developed by conventional electrophotographic techniques. The principal disadvantage of such a device is that only those X-rays which are absorbed within a few microns of a flat cathode surface can cause photoelectrons to be ejected into the gas gap. For example, in the case of a cathode formed of gold or lead, the effective X-ray photoelectron escape depth is less than 1000 A and the effective X-ray quantum absorption for such a thin layer is usually less than about 0.5%. This low effective X-ray quantum absorption value is maintained for flat cathodes fabricated of common photocathodic materials, including cesium iodide (CsI). As a result, the efficiency of a flat plate cathode is very poor and electrostatic X-ray images of good quality can be obtained from such a device only by utilizing high X-ray exposure dosages. Consequently, such a device is not practical for medical radiography where large X-ray dosages are harmful to the patient.
It is desirable to have an X-ray radiography device capable of reducing the X-ray exposure level to the patient while maintaining comparable radiograph quality and resolution.